Keyless entry control systems are well known examples of short-distance wireless communication systems. The keyless entry control system exercises wireless control, for instance, over vehicle door opening/closing, engine startup, and engine shutdown from a distance of several meters to several tens of meters. The system comprises a stationary wireless communication device, which is mounted in a vehicle, and a portable wireless communication device, which is carried by the user.
FIG. 8 is a block diagram that shows how a conventional keyless entry control system of the above-mentioned type is configured. In the example shown in FIG. 8, a transmitter 1, which is a wireless communication device carried by the user, modulates a carrier band signal having a VHF or higher frequency with an identification code for the transmitter 1 and an instruction signal for door opening/closing or the like, and transmits the modulated signal from an antenna 5 to a receiver 2, which is a stationary wireless communication device.
The receiver 2 includes an antenna filter for filtering the received carrier band signal to remove unnecessary frequency components. In the keyless entry control system, a 315 MHz high-frequency signal is used (hereinafter referred to as the “communication frequency”). Therefore, a surface acoustic wave filter 6 (hereinafter referred to as a SAW filter), which is suitable for use at high frequency, is generally employed as the antenna filter for the receiver 2.
According to Radio Law Enforcement Regulations, the electric field strength prevailing at a distance of 3 m from the transmitter 1 is not restricted if it is 500 μV/m. Since the carrier band signal is transmitted at low power, the receiver 2 uses a high-frequency preamplifier 14 to raise the received carrier band signal to a specified power level and demodulates the identification code and the instruction signal by subjecting the amplified carrier band signal to direct detection by reception mechanism 4. The instruction signal is then used to control the vehicle's drive mechanisms to, for instance, open/close a door.
The reception mechanism 4 can be integrated into an integrated circuit (IC). Therefore, the receiver 2 can be reduced in size by mounting the reception mechanism and SAW filter on a circuit board as so-called discrete parts.
In general, the relationship between the noise level N (W) and bandwidth B (Hz) of a receiver that is directly detected by an antenna filter is uniquely determined by Equation (1) below:N=KTBF  (1)
In Equation (1), the symbol K represents Boltzmann's constant (1.38×10−23); T, absolute temperature (degrees K); and F, noise index. Since Boltzmann's constant K, absolute temperature T, and noise index F are fixed values, the noise level N depends on the antenna filter's bandwidth B. In other words, the wider the bandwidth B of the employed antenna filter, the higher the noise level N and thus the lower the signal component reception sensitivity. Further, since the bandwidth B is wide, mutual interference with unnecessary extraneous signals is likely to occur, thereby causing improper operation.
FIG. 9 shows a frequency-attenuation characteristic that prevails when the SAW filter 6 is used as the antenna filter. It indicates that the bandwidth in a 3 dB attenuation region W1 is approximately 1000 kHz under normal conditions. If, for instance, the identification code bit rate is 1.2 kbps, the bandwidth required for communication (W2) is 4 kHz or smaller.
As described above, the SAW filter 6 provides a very wide bandwidth (approximately 1000 kHz) in marked contrast to the bandwidth required for communication (4 kHz). Therefore, it is substantially impossible to raise the signal component reception sensitivity by lowering the noise level N as is indicated by Equation (1). Thus, the range for establishing communication between the receiver 2 and the transmitter 1 (e.g., communication distance) is limited.
Further, the bandwidth B of the SAW filter 6 cannot be narrowed. It is therefore difficult to effectively prevent mutual interference between an extraneous signals and a communication frequency (i.e., the signal frequency required for communication), eliminate image frequencies for the communication frequency, and improve communication quality. To achieve image frequency elimination, it is necessary that the reception mechanism 4 incorporate an image suppressor function. Consequently, it is impossible to simplify the configuration of the reception mechanism 4 and reduce the power consumption.
The SAW filter 6 is relatively expensive because it is made by mounting a plurality of electrodes on a crystal plate surface. Further, the SAW filter 6 needs to be mounted on a set circuit board separately from the reception mechanism 4, which is integrated into an IC. Therefore, the size reduction of the receiver 2 is somewhat limited.
In a short-distance wireless communication system such as the aforementioned keyless entry control system, particularly in a system where a SAW filter 6 is used as an antenna filter, the single antenna filter is not used for both transmission and reception. One major reason is that the transmission frequency f1 and reception frequency f2, used for communication, differ from each other even when the same carrier band signal is used. Another major reason is that when a high-frequency signal that has not been filtered by the SAW filter 6 is amplified, the carrier band signal level outside the bandwidth required for communication increases to adversely affect the SAW filter 6 (e.g., the operation becomes unstable so that the specified filter characteristic is not obtained).
Therefore, it would be desirable to provide a small-size, high-frequency wireless communication device that is capable of enlarging the range within which communication can be established, effectively avoiding mutual interference with extraneous signals, and easily achieving image frequency suppression.
In view of the relationship between noise level and bandwidth, which is indicated by Equation (1) above, the present invention provides a high-frequency wireless communication device that employs a crystal filter as an antenna filter. The bandwidth provided by the crystal filter is much narrower than that provided by a conventional SAW filter. When the crystal filter is used, the bandwidth for carrier band signal passage can be reduced to 20 kHz or less whereas the SAW filter provides a bandwidth of approximately 1000 kHz. According to the law of energy conservation, the receiving-end reception sensitivity can therefore be dramatically raised in relation to the same energy (electrical power for transmission).
Conventionally, the crystal filter has been frequently used with a wireless communication device that is based on relatively low frequencies including a shortwave band. However, the use of the crystal filter with a VHF band (30 to 300 MHz) or UHF band (300 MHz to 3 GHz) has not been conceived. One major reason for this is that an upper limit is imposed on the range of the crystal filter implementation as indicated, for instance, in FIG. 7 on page 23 of “Crystal Device General Description and Application” (Quartz Crystal Industry Trade Association of Japan, March 2002). It was believed that the SAW device (SAW filter) developed for a high-frequency band is the only applicable antenna filter for a keyless entry control system for use at a VHF or higher frequency.
However, due to improvements in manufacturing and processing technologies for crystal chips comprising the crystal filter the range of frequencies applicable to the crystal filter has been enlarged. Further, crystal can be used not only at a fundamental frequency but also at an overtone frequency, which is an odd-numbered multiple of the fundamental frequency. Thus, it has been found that the crystal filter (crystal chip) can also be used as an antenna filter at VHF and higher frequencies.